API Documentation

The calculator's REST API is public, requires no authentication, and returns the same numbers the web form would. Anonymous traffic is throttled at 60 requests per minute per IP. A higher-volume authenticated tier will arrive with the B2B widget rollout.

Endpoints

Choose an endpoint that matches the calculator you'd use on the website:

Endpoint	Calculator
`POST /api/calculate/paye/`	Standard PAYE (employee salary)
`POST /api/calculate/limited_co/`	Limited Company — director's salary + dividends
`POST /api/calculate/umbrella/`	Umbrella Company — day rate to PAYE
`POST /api/calculate/ir35/`	IR35 inside vs outside comparison
`POST /api/calculate/sole_trader/`	Sole Trader / Self-Employed — Class 4 NI + Self Assessment
`GET /api/tax-rates/`	Read-only inventory of seeded tax-year rate snapshots

Status codes

200 — calculation successful, JSON body matches the result table for that calculator.
400 — validation error (unknown mode, invalid tax code, missing required field, zero day rate, etc.). Body: {"error": "…"}.
404 — the requested tax_year isn't seeded in this deploy.
429 — throttle limit hit (60 req/min per IP).

Examples

PAYE

curl -X POST https://tax.log7.uk/api/calculate/paye/ \
    -H "Content-Type: application/json" \
    -d '{
          "tax_year": 2025,
          "income": 45000,
          "income_type": "yearly",
          "workweek_hours": 40,
          "tax_code": "1257L",
          "pension_scheme": "net_pay",
          "pension_percent": 5
        }'

Limited Company

curl -X POST https://tax.log7.uk/api/calculate/limited_co/ \
    -H "Content-Type: application/json" \
    -d '{
          "tax_year": 2025,
          "lc_revenue": 100000,
          "lc_expenses": 0,
          "lc_director_salary": 12570,
          "lc_dividends": 30000
        }'

Umbrella

curl -X POST https://tax.log7.uk/api/calculate/umbrella/ \
    -H "Content-Type: application/json" \
    -d '{
          "tax_year": 2025,
          "umb_day_rate": 500,
          "umb_working_days": 220,
          "umb_margin_per_week": 25,
          "umb_tax_code": "1257L"
        }'

IR35 inside vs outside

curl -X POST https://tax.log7.uk/api/calculate/ir35/ \
    -H "Content-Type: application/json" \
    -d '{
          "tax_year": 2025,
          "ir35_day_rate": 500,
          "ir35_working_days": 220,
          "ir35_agency_margin_per_week": 0,
          "ir35_outside_expenses": 0,
          "ir35_outside_director_salary": 12570,
          "ir35_small_client": false,
          "ir35_tax_code": "1257L"
        }'

Sole Trader

curl -X POST https://tax.log7.uk/api/calculate/sole_trader/ \
    -H "Content-Type: application/json" \
    -d '{
          "tax_year": 2025,
          "st_revenue": 30000,
          "st_expenses": 0,
          "st_paye_income": 0,
          "st_pay_class_2_voluntarily": false,
          "st_is_scotland": false,
          "st_tax_code": "1257L"
        }'

Field naming

Request body field names mirror the corresponding HTML form field names. Each calculator uses a short prefix to keep them disambiguated:

PAYE — no prefix (income, tax_code, …)
Limited Co — lc_ (lc_revenue, lc_director_salary, …)
Umbrella — umb_
IR35 — ir35_
Sole Trader — st_

Notes & gotchas

The API does not apply web-form defaults (e.g. PAYE pension defaulting to "Net Pay + 5%"). API consumers pass exactly the values they want.
Unauthenticated traffic is rate-limited; build clients that respect Retry-After headers when they hit a 429.
Tax-year dataset is seeded on deploy. GET /api/tax-rates/ tells you which years are available. Asking for a non-seeded year returns 404.

Caveats

The API is not for production traffic in its current state — no SLA, no guaranteed uptime beyond best-effort, no notice on breaking changes. See the Terms of Use for the liability disclaimer that applies to API output as well.